1. Field of the Invention
This invention relates to electrodeposition of chromium, and, more particularly, it is concerned with an activation process by which an adherent chromium electrodeposit can be formed on stainless steel substrates from a high energy efficient chromium plating bath.
2. Description of the Prior Art
Commercial use of high energy efficiency chromium plating baths has been hampered by their inability to provide adequate coating adhesion to certain metal substrates. The baths themselves are disclosed in Mitsui, J7B-33941 (Sept., 1978): Dillenberg, U.S. Pat. No. 4,093,522: Perakh et al., U.S. Pat. No. 4,234,396: and Chessin, U.S. Pat. Nos. 4,450,050 and 4,472,249.
The use of sulfuric acid and hydrofluoric acid etches for stainless steel substrates to improve adhesion has been recommended for chromium deposition. For example, a table which gives suitable lengths of time for various substrates for such an etching process is found in "Metal Finishing" 80 (5) pages 65-8 (1982) by C. H. Peger.
Anodic chromic acid etching treatments for 400 stainless steel alloys and for low and high carbon steels is disclosed in "48th Metal Finishing Guidebook-Directory" 78, 88-202 (1980) by A. Logozzo. Also recommended are cathodic treatments in sulfuric acid-fluoride solutions for 300 stainless, for nickel alloys and for cast iron.
The use of a slight reverse in the plating bath after reversing in sulfuric acid is disclosed at page 136 of "Hard Chromium Plating" Robert Draper Ltd., Teddington, England (1964) by J. D. Greenwood.
ASTM B177-68 describes the use of sulfuric acid or chromic acid as an activator for chromium electroplating on steel for engineering use.
Chessin in U.S. Pat. No. 4,450,050 describes an activation pretreatment for bonding high efficiency chromium electrodeposits on a metal substrate which is characterized by the pre-step of plating the substrate metal with iron or an iron alloy from an iron salt containing bath.
Herrmann, in U.S. Pat. No. 4,416,758, activates metal substrates in an aqueous alkaline cyanide containing solution using current which is periodically reversed, followed by rinsing and chromium plating.
Chen and Baldauf, in U.S. Pat. No. 4,412,892, uses an anodic treatment in a sulfuric acid-hydrochloric acid solution.
Electroplating Engineering Handbook, p. 167 Graham (Rheinhold) N.Y. (1955) suggests a dip in 1% sulfuric -0.1% hydrochloric solution, or an anodic treatment in the plating bath itself.
It has been found that when these procedures are employed with high energy efficient chromium plating baths, stainless steel substrates still are not adequately plated because of poor adhesion. It can be speculated that the reducing conditions at the cathode during initiation of deposition causes the halide ion present in the high energy efficient bath to be reduced to a form which interferes with the molecular bonding of chromium to the substrate. In any event, the use of high efficiency chromium plating on stainless steels has been limited by the problem of inadequate adhesion.
Accordingly, it is an object of this invention to provide an improved process for forming adherent, bright, smooth and hard chromium electrodeposits, particularly from high energy efficient chromium baths, on stainless steel substrates.
Still another object is to provide an activation solution for electrolytically etching a stainless steel metal substrate in preparation for electrodepositing an adherent, bright, smooth and hard chromium metal deposit thereon from a high energy efficient chromium bath, where the degree of adherence of the chromium deposit can be predetermined by the composition and concentration of the solution, and conditions of etching.